In the technique of continuous-ink-jet printing, such as first described in U.S. Pat. No. 3,369,252, a stream of liquid ink is subdivided into a sequence of small droplets by a piezoelectric resonator, vibrating at a frequency of the order of 50 kHz or more. The droplets are projected out of a nozzle towards the substrate. The electric potential against ground of said nozzle, and thus the electric charge of the resulting ink droplets, is controlled by an addressing electrode. Two deflector electrodes, arranged along the projection path of said ink droplets, are held on predefined, constant electric potentials. They deflect the ink droplets according to their electric charge and make them impact on different locations on the substrate. By applying a corresponding electric pulse of appropriate voltage and duration to the addressing electrode, a single ink droplet can be addressed and directed to a specific location on the substrate. In the absence of an addressing voltage, the projected ink droplets are collected by a small collecting funnel, from where they are recycled to an ink reservoir. Refinements to the described printing method and equipment include the use of uncharged or slightly opposite charged guard drops between the printing droplets, such as first proposed in U.S. Pat. No. 3,562,757, in order to minimize the effects of the mutual repulsion between droplets having the same electric charge. Further refinements to the printing method are proposed in U.S. Pat. No. 3,828,354; U.S. Pat. No. 3,833,910; U.S. Pat. No. 3,846,800; U.S. Pat. No. 4,688,048; as well as in many other publications.
Inks for the continuous-ink-jet printing are normally of the soluble-dye type, in order to provide for the necessary homogeneity, and thus, an easy runnability on the printing machine. The inks may furthermore be formulated as solvent-based or water-based liquids. The later option is increasingly preferred, due to the ever growing environmental restrictions which apply to the use of organic solvents. However, printings made with such inks lack in general resistance against environmental influence, and they are also affected by solvents and/or water. Waterfast inks in particular are highly desirable for the production of permanent indicia or images.
Pigmented inks for the continuous-ink-jet printing are known in the art and first described in U.S. Pat. No. 4,365,035. More recent publications include EP 751,194 and EP 853,106. Particular technical problems arise from the heterogeneous nature of the ink, which must be solved through the use of particular dispersing additives, and, principally, through a continuous stirring and circulating of the ink during printing operation.
Pigment particles for ink-jet printing are preferably smaller than 1 micrometer, more preferably even smaller than 300 nanometers. However, the presence of some particles as large as 5 micrometers can be tolerated in an ink-jet formulation. In particular, ink-jet inks are generally passed through a filter of 10 micrometer pore size, in order to eliminate pigment fractions or agglomerates larger than this, which could obstruct the printing nozzles.
Existing solutions to maintain the pigmented ink dispersed during printing operation, and to prevent hard settling of the pigment between printing operations, include the following approaches:                In U.S. Pat. No. 6,062,682 the ink cartridge is moved along a carrier shaft such as to stir the pigment, and in U.S. Pat. No. 5,451,987 the ink- and recovery-reservoirs are equipped with means, such as a magnetic stirrer, for agitating the ink.        U.S. Pat. No. 5,650,802 suggests the use of a vibrating piezo element to prevent the pigment suspension from settling outside the printing periods.        Alternatively, a re-circulation of the ink in the system at regular frequency has also been successfully used to maintain the ink in an appropriate state of dispersion. Such a system is currently exploited in the OPAQUE DOMINO CIJ printers.        
In the field of security printing, it would be highly desirable to have available ink-jet inks which resist to the influence of light and to commonly available chemical agents, such as household bleach or solvents of the most various kind. Such inks could noteworthy serve for the rapid and reliable on-line numbering of security documents, such as banknotes, identity documents, lottery tickets, warranty labels and the like.
A particular type of waterfast security inks for the ink-jet numbering of lottery tickets has been disclosed in U.S. Pat. No. 5,498,283. However, the disclosed inks do merely contain an organic dye which has improved resistance to bleach; formulations containing inorganic luminescents are not disclosed.
To provide for an appropriate intensity of the luminescence response, inorganic security pigments have generally a particle size of greater than 300 nm. Example of such pigments can be found in the disclosures of U.S. Pat. No. 5,891,361; U.S. Pat. No. 6,132,642 and WO 00/63317. Below said particle size, the surface-to-volume ratio of the particles, and thus the influence of the surface defects responsible for luminescence quenching, becomes too important, resulting in a strong drop of the luminescence quantum yield.
The addition, to a continuous ink jet ink, of inorganic luminescents of said mentioned types noteworthy presents a couple of particular problems, which are due to the relatively high specific weight of said inorganic compounds, as compared to that of the printing liquid, to the required pigment size, as well as to the very low viscosity of the printing fluid. This results in a strongly increased tendency of the pigment to sediment, forming furthermore relatively dense deposits which are difficult to redisperse. After a stop and restart of the printing operations, not all pigment is thus necessarily redispersed; some of it remaining settled in the corners of the ink reservoir. Moreover, hard sedimentation of the inorganic pigment, when occurring between printing operations or during prolonged storage periods, is very difficult to redisperse and requires high shear mixing forces to put the pigment back into suspension.
Hard sedimentation results in a change in concentration of the active ingredient with time, which is not compatible with the stringent standards of security printing.
The combination of a particularly enhanced stirring and circulating of the printing liquid during operation, together with the use of an ink formulation according to the present invention, are efficiently used to remedy this problem.